This invention relates generally to protective relays used in power systems, and more specifically concerns a new delta filter for use in determining incremental changes in voltage and currents on a power line.
For at least the past 20 years, protective relays for power systems have used what are generally referred to as delta filters (also referred to as xe2x80x9cxcex94xe2x80x9d filters) to measure change in voltage and current quantities on a power line due to faults/disturbances on the line. In such applications, delta filters are responsive to voltage or current time-varying waveforms from the power line, and in operation subtract the waveform present at a selected interval of time prior to the present time from the present time waveform. This is accomplished by a delay characteristic (capability) of the filter. The selected interval of time is equal to a selected integral multiple of the time-varying voltage/current waveform time period. In most of the early delta filter applications, the delay was one power system cycle.
Such delta filters are quite secure under steady-state conditions. With no change in the waveforms when there is no disturbance or fault event, the output of the delta filter will be zero. Then, when an event or fault on the power line occurs, resulting in a change in the current or voltage waveform, the delta filter will have a non-zero output, the magnitude of which is indicative of the significance of the actual change in the power signal system as represented by the voltage and/or current values.
An example of this initial type of delta filter is shown in FIG. 1. Referring to FIG. 1, the filter, shown generally at 10, is responsive to a sinusoidal voltage v(t), which is shown, or current i(t) signal obtained from the power line. The same filter technique can be used with other quantities, including frequency or distance measurements. The present or immediate time value of the input signal is then subtracted by a subtract or difference element 12 from a time delayed signal, to provide an output xcex94v(t) or xcex94i(t), which is the difference between the two signals applied to the difference element 12. A time delay element 14 produces the delayed signal on a continuous basis, with a time interval between the delayed signal and the present time signal equal to a selected multiple of the period of the input signal. The delay produced by the element 14 is referred to as the delta filter time-window. The delayed signal from time element 14 is referred to as the reference signal. The Laplace transform representation of the delay is e-nTs, where n is the selected multiple of the input signal period, T is the input signal period and s represents a standard Laplace mathematical operator.
In the development of delta filters, phasor quantities have been used as inputs to the delta filter, rather than time-varying input waveforms from the power line. Positive sequence voltage and current phasors are often used in such embodiments. In one possible embodiment, rotating phasors are used, while in another embodiment, the phasors are time-invariant. Both systems require an input filtering system to produce the desired phasors for the delta filter system.
An example of such an input filtering system is a finite-response pair of orthogonal filters which produce a phasor output which rotates counterclockwise in the complex plane by an angle equal to 360xc2x0 divided by the number of samples N per cycle acquired from the waveform v(t) or i(t) from the power line. Under steady-state power system conditions, the rotating phasor output has the same coordinates at every multiple of the waveform period. In order to be able to subtract two phasors in a delta filter, the delta filter time window must be equal to an integral multiple of the incoming signal period. The rotating phasor can be made time-invariant by multiplying (in the filter system) the Fourier filter output by the same angle rotating in the clockwise direction.
A time-invariant phasor does not change its position with time in the complex plane, unless the frequency of the incoming signal undergoes some changes. The incoming (present) phasors and the time-delayed phasors in the time-invariant arrangement will have exactly the same coordinates, if there is no change on the electrical network, and will hence produce a zero delta filter output. One advantage with such a system is that the time window for the time-invariant phasor delta filter can basically assume any value. A time delay of a selected amount between 0.5 and 3 cycles can be typically accommodated by a delta filter. With such a system, the filter output will be zero when the time-invariant phasor does not change, i.e. for when there is no change in the electric power signals on the line. The output of the filter will be other than zero when there is a disturbance due to a fault on the line.
All of the above-described delta filter approaches, however, have difficulty in those situations where the fault or disturbance on the power line changes with time, i.e. an evolving fault, such as from a single-phase-to-ground fault (A-ground) to a different type of fault, e.g. an A-B-ground fault. When a single line-to-ground fault occurs in the power system, the delta filter associated with the particular faulted phase (line) will produce a non-zero sinusoidal output for a particular interval of time equal to the delta filter time window.
When the fault evolves, however, the reference signal for the delta filter for the non-faulted phase will be contaminated with fault quantities related to the original fault. This leads to inaccurate results relative to the determination of the evolving fault.
When successive network power changes occur (an evolving fault), detection with a delta filter is as a result made with a reference phasor which is not stationary in time, and the resulting outputs, which are obtained at different times, cannot be correlated because of the differences in the reference signal. The detection of any type of evolving fault, where the reference for another phase is contaminated by a previous condition such as a fault, leading to the successive changes in the incoming signal to the delta filter, is affected. One example is a forward fault which changes into a reverse fault. Another example, as discussed above, is a single phase-to-ground fault which evolves to a phase-to-phase-to-ground fault. It would be desirable to have a delta filter system in which evolving faults could be accurately detected by solving the problem of the changing reference phasor.
Accordingly, the present invention is a delta filter system for use in a protective relay for power systems, comprising: an input portion for receiving electrical signal values representative of selected electrical quantities present on a power line which change in response to a change in the power system condition; a first comparison element for comparing said electrical signal values at a present point in time with said electrical signal values at a selected past point in time, i.e. an earlier point in time, wherein the selected past point in time values are provided by a delay element and function as reference values; circuit means for providing a first incremental signal quantity output if the past and present values are different, the existence of a first incremental signal quantity being indicative of a change in the condition of the power system, possibly a fault; a memory for storing the present time electrical signal value following the appearance of said first incremental signal quantity output; and means using said stored signal value as a fixed reference value for future comparisons in a second comparison element beyond the point in time of the appearance of said first incremental signal quantity output to produce a second incremental signal quantity output if the power system continues to change.